Cone Idioblasts of Eleven Cycad Genera: Morphology, Distribution, and Significance

Abstract

Sporophylls from strobili of 16 species among 11 genera of cycads were examined for idioblasts. Thin-walled secretory-like idioblasts were found in the majority of taxa and thick-walled sclerenchyma-type idioblasts were found in the minority. With the exception of Cycas rumphii and Stangeria eriopus, where secretory idioblasts were in the epidermis and/or hypodermis, none were found in the sporophyll parenchyma of these species. Most idioblasts stained ninhydrin-Schiff-positive, and tannins were present also. Sporophyll idioblasts appear to be related to interactions with insect predators and/or cosymbionts and may form part of a complex pollination syndrome. The lack of idioblasts in stem tissue and low concentration in leaflet tissue of Zamia furfuracea compared with sporophyll tissue is significant support to this hypothesis. On the basis of almost universal occurrence of these idioblasts in the sporophylls, we suggest that insect symbiosis related to pollination may be common to most cycad genera.